Shunt stabilization of welder

ABSTRACT

This invention discloses a resistance welding system involving a power level between 2kw. and 100kw. suitable for use in semiconductor lead welding applications where the total circuit impedance is three or more times that of the weld area and the current is in the range of 2,000 to 15,000 amperes or higher. The welding system features a shunt around the weld area where the shunt resistance is comparable to the resistance of the weld area.

United States Patent Inventor Joseph T. Wallace Phoenix, Ariz.

Appl. No. 801,934

Filed Feb. 25, 1969 Patented Apr. 27, 1971 Assignee Motorola, Inc.

Franklin Park, Ill.

SHUNT STABILIZATION OF WELDER 4 Claims, 1 Drawing Fig.

U.S.Cl 219/108 Int.Cl B23k9/10 FieldofSearch 219/108- [56] ReferencesCited UNITED STATES PATENTS 2,238,419 4/1941 Hass 2l9/l08 PrimaryExaminer-J. V. Truhe Assistant Examiner.l. G. Smith Attorney-Mueller &Aichele ABSTRACT: This invention discloses a resistance welding systeminvolving a power level between 2kw. and l00kw. suitable for use insemiconductor lead welding applications where the total circuitimpedance is three or more times that of the weld area and the currentis in the range of 2,000 to 15,000 amperes or higher. The welding systemfeatures a shunt around the weld area where the shunt resistance iscomparable to the resistance of the weld area.

DC VOLTAGE S0 RCE Patented A ril 21, 1971 DC VOLTAGE SOURCE mvsmoaJoseph 7. Wallace BY 77%, ddclwie, f fauna SHUNT STABILIZATION F WELDERThis invention relates to a welding system and more particularly to awelding system adapted to weld a plurality of low re sistancesemiconductor leads to a low resistance metal frame member.

The simultaneous welding of a plurality of low resistance leads, forexample 14, of a semiconductor device to other low resistance metalleads utilizing commercially available welders results in a number oferratic welds and electrode failures. It is believed that these erraticwelds and failures are a result of buildup of aluminum on the weldingelectrodes or caused by the presence of dirt, epoxy, or othercontaminants on the metal leads. The aluminum buildup on the electrodesand/or the presence of dirt or epoxy on the leads increases theresistance of the weld area. When the resistance of the weld areaincreases, the voltage drop increases correspondingly and as a result,the power or energy in the weld area, being proportional to the voltagein this instance, increases. It is this power or energy increase whichcauses the erratic welds which take the form of burned up workpieces andelectrodes.

It has been widely accepted that the welding current through theworkpiece being welded must be accurately controlled during a weldingcycle. Welding apparatus of the type disclosed in US. Pat. No. 3,275,790employ a dynamic system involving a power level of less than 2, and acurrent in the order of 100 to 1,000 amperes. In a dynamic system suchas this, the current regulator adjusts the voltage going to the weldingelectrodes based upon the sensed voltage generated at the weldingelectrodes. This dynamic system is suitable for welding one or two leadsat one time since the current or energy output is relatively low.However, dynamic systems of this type are designed to operate onamperages which are substantially lower, and hence are not suited forhigh current welding systems which require between 2,000 to l5 ,000 ormore amperes.

Commercially available welders which are capable of sup plying anamperage in the range of 2,000 to l5,000 amperes that is necessary tosimultaneously weld to 14 leads at one time have a total circuitimpedance which is three or more times that of the weld area. In systemsof this type involving power levels between 2 and 100, it becomes tooexpensive to attempt regulation of the weld area voltage by ordinarymeans such as the type referred to in the aforementioned patent. It isthe practice in high power levelwelding systems of this type to relyupon quality control of the work and electrode surfaces, that is,maintain a substantially constant work area resistance by keeping thework and the electrodes very clean. In the mass production welding ofsemiconductor leads, it is difficult at best to maintain such rigidcontrol over the cleanliness of the work and electrodes.

It is an object of this invention to provide an improved 2 to 100, powerlevel welding system.

It is another object of this invention to provide a welding system using2,000 to 15,000 amperes suitable for use in making multiple weldssimultaneously.

It is another object of this invention to control the energy through theworkpiece during a welding cycle.

It is yet another object of this invention to reduce erratic welds.

It is yet another object of this invention to avoid destruction of theworkpiece when the workpiece has a resistance higher than normallyencountered.

These and other objects are accomplished by connecting a shunt aroundthe weld area. The resistance of the shunt is similar to or up to twiceas great as the resistance of the weld area. In a welding system havinga shunt with a resistance similar to the resistance of the weld area andconnected thereto, a change in the resistance of the weld area willresult in very little change of the weld area energy. For example, anincrease of the weld area resistance of 50 percent results in the weldarea getting 40 percent of the current instead of the nominal 50percent, a decrease of percent current. At the same time, the totalresistance of the weld area and shunt has increased by percent resultingin a voltage increase of about 22 percent. The energy, which is theproduct of the current and the voltage, is substantially the same sincethe lower current and the higher voltage values offset one another. Thisarrangement permits the weld to be made over a wider range of workpieceresistance.

Other objects and advantages of this invention will be apparent from thefollowing detailed description, reference being made to the accompanyingdrawings wherein a preferred embodiment of this invention is shown.

Referring to the drawing, the resistance weld system 10 has weldingelectrode heads 12 in contact with thin metal leads l4 and 16 which areto be welded together. The resistance of the leads 14 and 16 which formthe workpiece are of low resistance metals such as aluminum, gold, andthe like. Since the workpiece is of low resistance metals, it isdifficult to have a work area resistance which approaches one-half ormore of the resistance of the secondary circuit, a desirable resistancefor efficient use and for control purposes. One of the electrode heads12 is a cylindrical electrode and the other electrode head is a flatanvil. In the preferred embodiment the cylindrical electrode head comesin physical contact with 14 individual sets of leads which are to bewelded. The leads 1 4 and 16 establish electrical contact between thetwo electrode heads.

Flexible leads 18 are connected to electrode heads 12. A weldingtransformer 20 has a primary winding 22 and a secondary winding 24.Leads 18 are connected to the transformer secondary winding 24. Primarywinding 22 is connected to a switch 28, a power source, and a capacitor30.

In accordance with this invention, a shunt 32 is connected to the endsof the weld area, that is, electrode heads 12. The shunt has aresistance which is about the same as the resistance of the work areawhich includes the electrode heads 12 and the leads l4 and 16. Theresistance of the shunt may vary from 0.5 to 2.0 times the resistance ofthe work area with the preferred shunt resistance being equivalent tothe work area resistance.

In a preferred embodiment of this invention, a commercially availablewelder was used which was capable of providing a current of 12,000 amps.The resistance of the weld area between electrode heads I2 is about 70microhms. The resistance of the leads l8 and the resistance in thesecondary winding of the transformer 24 is about 700 microhms. Theresistance of the weld area is about 8 percent of the total resistancein the circuit. In accordance with the practice of this invention, theshunt 32 has a resistance of about microhms. The presence of a shuntprevents the energy or power level from increasing at the electrodeheads 12 and in the weld area 14 and 16 when the resistance thereofincreases due to aluminum buildup on the electrodes or to dirt or epoxyon the leads l4 and 16. When the weld area resistance increases, thevoltage will increase correspondingly but the current will diminish dueto the presence of the shunt. As a result, the total power in the weldarea, the product of the voltage, and the current will remain fairlyconstant. The use of a shunt 32 limits the energy across the weldterminals 12 and eliminates catastrophic weld failures.

lclaim:

l. A high current resistance welding system having a total circuitimpedance which is at least three times that of the weld area undernonnal preweld conditions comprising a weld area and a shunt, said weldarea consisting of a workpiece to be welded, a first electrode inelectrical contact with said workpiece, and a second electrode inelectrical contact with said workpiece, said shunt connected to saidfirst electrode and to said second electrode and having under normalpreweld conditions a resistance equal to 0.5 to 2.0 times the resistanceof said weld area.

2. A resistance welding system described in claim I wherein theresistance of said shunt is about I to 1.5 times the resistance of saidweld area.

3. A welding system as described in claim 1 wherein the resistance ofsaid shunt is about 105 microhms and the resistance ofsaid weld area isabout 70 microhms.

4. A welding system as described in claim I wherein said welding systemhas a total circuit impedance of from 700 to 800 microhms.

1. A high current resistance welding system having a total circuitimpedance which is at least three times that of the weld area undernormal preweld conditions comprising a weld area and a shunt, said weldarea consisting of a workpiece to be welded, a first electrode inelectrical contact with said workpiece, and a second electrode inelectrical contact with said workpiece, said shunt connected to saidfirst electrode and to said second electrode and having under normalpreweld conditions a resistance equal to 0.5 to 2.0 times the resistanceof said weld area.
 2. A resistance welding system described in claim 1wherein the resistance of said shunt is about 1 to 1.5 times theresistance of said weld area.
 3. A welding system as described in claim1 wherein the resistance of said shunt is about 105 microhms and theresistance of said weld area is about 70 microhms.
 4. A welding systemas described in claim 1 wherein said welding system has a total circuitimpedance of from 700 to 800 microhms.